1. Field of the Invention
The present invention relates to a hydraulic control apparatus for controlling a hydraulically operated continuously variable transmission of belt-and-pulley type used for a motor vehicle.
2. Discussion of the Prior Art
A belt-and-pulley type continuously variable transmission whose speed ratio is continuously variable is known as a transmission incorporated in a power transmitting system for a motor vehicle. This continuously variable transmission includes a first and a second shaft, a pair of variable-diameter pulleys provided on the first and second shafts, respectively, a belt connecting the pulleys for transmitting power therebetween, and a pair of hydraulic actuators for changing effective diameters of the pulleys. An example of a hydraulic control apparatus for such a belt-and-pulley type continuously variable transmission is disclosed in laid-open Publication No. 64-49755 of unexamined Japanese Patent Application. In the hydraulic control apparatus disclosed in this publication, the line pressure of the hydraulic system for controlling the tension of the transmission belt of the transmission, i.e., the belt tensioning pressure, is regulated so as to change with the speed ratio of the transmission, by a pressure regulating valve. The pressure regulating valve regulates the belt tensioning pressure, based on a required output pressure and a speed-ratio pressure, such that the rate of change in the belt tensioning pressure varies along a straight line which is bent at a point corresponding to a specific value of the speed ratio of the transmission. Namely, the rate of change in the belt tensioning pressure changes when the speed ratio reaches a certain value. The required output pressure represents the currently required output of the engine, while the speed-ratio pressure represents the currently detected speed ratio of the transmission. The hydraulic control apparatus including such a pressure regulating valve is comparatively economical to manufacture and reliable in operation, while assuring adequate regulation of the belt tensioning pressure such that the belt tensioning pressure almost follows an optimum relationship with a change in the speed ratio.
The above-indicated optimum relationship between the belt tensioning pressure (tension of the belt) and the speed ratio of the transmission is desirably formulated to maintain the belt tension at a required minimum level. In this case, the optimum relationship is represented by a curve, rather than a bent straight line which the pressure regulating valve of the known hydraulic control apparatus substantially follows as described above. When the belt tensioning pressure is regulated along the straight line rather than the optimum curve, the belt tensioning pressure tends to be unnecessarily higher than the optimum level under certain conditions, resulting in a failure to effectively minimize the power loss of the oil pump of the transmission hydraulic system which is driven by the engine of the vehicle.